As the conventional side seamless can, there can be mentioned a product formed by subjecting a metal blank such as an aluminum plate, a tinplate or a tin-free steel plate to an ironing operation in at least one stage between a drawing die and a punch to form a cup comprising a side seamless barrel and a bottom connected integrally and seamlessly to the barrel, and if desired, subjecting the barrel to an ironing operation between an ironing punch and an ironing die to reduce the thickness of the barrel of the vessel. It also is known that the thickness of the side wall can be reduced by bending-elongation by a curvature corner portion of the redrawing die instead of the above-mentioned ironing operation (see Japanese Unexamined Patent Publication No. 56-501442).
In order to save a vessel material and prepare a can having a large can height from a certain quantity of a metal blank, it is preferred that the thickness of a barrel portion of a vessel be reduced by ironing or by bending elongation.
As the method for forming an organic coating on a side seamless can, there can be mentioned not only a method broadly adopted in the art, in which an organic paint is applied to a formed can, but also a method in which a resin film is laminated on a metal blank before the forming. As an example of the latter method, Japanese Examined Patent Publication No. 59-34580 proposes a method in which a metal plate laminated with a film of a polyester derived from terephthalic acid and tetramethylene glycol is used. It also is known that in the production of a redraw-formed can by bending elongation, a metal plate coated with a vinyl organosol, epoxy, phenolic, polyester or acrylic paint can be used.
However, the conventional method for the production of a coated can having a thickness-reduced side wall is defective in that the corrosion resistance of an organic coating applied to a metal plate in advance is drastically degraded by the forming operation.
More specifically, the organic coating is readily damaged by a tool diring the side wall thickness-reducing forming operation, apparent or latent exposure of the metal is caused at the damaged part of the coating, and elution of the metal or corrosion is caused from this damaged part. In the production of a seamless can, there is produced such a plastic flow as increasing the size in the height direction of a can and diminishing the size in the circumferential direction of the can, and by this plastic flow, the adhesive force between the surface of the metal and the organic coating is reduced, and furthermore, this adhesive force is reduced with the lapse of time by the residual strain in the organic coating. This tendency becomes conspicuous when a content is hot-filled in the can or the filled can is subjected to heat sterilization at a low or high temperature.
It is therefore a primary object of the present invention to provide a side wall thickness-reduced deep-draw-formed can from a metal plate having an organic coating, which has an excellent coating completeness, an excellent adhesion of the coating, an excellent corrosion resistance and an excellent heat resistance.
Another object of the present invention is to provide a side wall thickness-reduced deep-draw-formed can, in which although the can is prepared from a metal plate having an organic coating, the adhesive force between the organic coating and the metal surface is prominently improved and as the result, an excellent combination of corrosion resistance and heat resistance is attained.